Formation of Peptizable Boehmites by Hydrolysis of Aluminum Nitrate in Aqueous Solution

JOURNAL OF COLLOID AND INTERFACE SCIENCE 188, 257–269 (1997)ARTICLE NO. CS974780

Formation of Peptizable Boehmites by Hydrolysisof Aluminum Nitrate in Aqueous Solution

EDISSON MORGADO, JR., 1 YIU LAU LAM, AND LINDA F. NAZAR*

Petrobras S.A./CENPES, Research and Development Centre, Cidade Universitaria, Quadra 7, Rio de Janeiro, RJ 21949-900, Brazil; and*Department of Chemistry, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada

Received April 29, 1996; accepted January 14, 1997

display some properties that are completely different fromMicrocrystalline boehmites were synthesized by aging amor- the well crystallized material.

phous precursors derived from base hydrolysis of aluminum ni- Microcrystalline boehmites are of primary interest be-trate solutions at 857C. The susceptibility of the resultant boehmite cause of their technical importance in the preparation ofgels to form a colloidal dispersion by reaction with dilute nitric

coatings and controlled porosity catalysts with enhancedacid, i.e., their peptizability, was determined based on their particlephysical properties (3–10). This is due to the susceptibilitysize measured by dynamic light scattering. Three different classesshown by some boehmite gels to peptization by dilute acidsof peptizabilities were obtained by modifying parameters duringat room temperature. Aluminum trihydroxides are not ex-the neutralization step and were related to the characteristics ofpected to have this property (3, 4) . Boehmite gels or xero-the boehmite gel and its precursors before aging. X-ray diffraction,

thermal analysis, solid state 27Al NMR, and transmission electron gels (powders) can be peptized to form either metastablemicroscopy techniques were applied to the characterization of the sols or less homogeneous pastes (high solid to liquid ratio) .materials. The preparation of peptizable boehmites was favored In the latter, the plastic properties of the molded mass andby low temperature hydrolysis where the amorphous AlO(OH) compacting of its structure is assured, and thus the desiredprecursor, after aging, readily converted to boehmite crystallites pore structure and mechanical strength of the final aluminathat did not aggregate into secondary particles. Nonpeptizable product can be achieved (5–7). On the other hand, colloidalboehmites were obtained at hydrolysis ratios (OH/Alú 3) at high

dispersions produced by acidic peptization of boehmite sus-temperature, leading to boehmite crystallites which condensed topension are required for a sol–gel process that yields porouslarge strongly interconnected polycrystalline fibrils. Partly peptiza-solid spheres (4, 8–10). In most cases, the relationship be-ble boehmite was obtained at high temperatures but lower OH/Altween peptization conditions and textural /physical charac-ratios. Under this condition, the fast crystallization of boehmiteteristics of the final product has been studied empirically.in the neutralization step was retarded, thus hindering strong in-

tercrystallite aggregation. This could be correlated to the presence The particles of the boehmite gel, as produced, are ag-of a significant portion of tetrahedrally coordinated aluminum in glomerates of crystallites. The homogeneous dispersion ofthe precursor. A brief discussion on the mechanisms of formation these aluminas is achieved by reducing the agglomerates toof the aluminum gel precursors is also presented. q 1997 Academic a sufficiently small particle size such that the particles remainPress suspended as a colloid. For some boehmites this is readily

accomplished by using noncomplexing, usually monovalentacids such as nitric acid (3, 4, 11). This process called

INTRODUCTION ‘‘peptization’’ is essentially a chemical disaggregation(breakdown) of large particles. The degree of peptization

For many years ‘‘well crystallized’’ boehmite (AlOOH in an aqueous boehmite dispersion should be viewed as aor Al2O3rH2O) was believed to be a material different reduction in the average particle size to the submicrometerthan the microcrystalline or ‘‘pseudoboehmite’’ form range, and can be measured, for example, by dynamic light(Al2O3rH2O, 1 õ n õ 2.5) . However, recent studies have scattering (DLS).showed that both have the same physical and chemical The synthesis of boehmites by neutralization of aluminumstructure, but differ in water content owing to their relative salts has been extensively studied in the literature (12–16),crystallite size (1, 2 ) . Despite this microstructural similar- but only a few reports refer to the peptizability of the materi-ity, synthetic microcrystalline boehmites (boehmite gels ) als. There is a surprising dearth of information on the rela-

tionship between the synthesis conditions and the peptizationability of the resulting boehmite gel. Even more scarce are1 To whom correspondence should be addressed. E-mail: emorgado@

cenpes.petrobras.com.br. studies that elucidate the mechanisms that lead to a ‘‘peptiza-

257 0021-9797/97 $25.00Copyright q 1997 by Academic Press

All rights of reproduction in any form reserved.

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258 MORGADO, LAM, AND NAZAR

ble’’ boehmite, i.e., that which is easily peptized by acidifi- TABLE 1Experimental Parameters for the Preparation of the Alumi-cation.

num Hydroxide Precursors and the Corresponding SyntheticThe present work is part of an investigation of the synthe-Boehmitessis of boehmites from hydrolized aluminum nitrate solutions

and the possible mechanisms of their formation. The boeh-Reaction Sample codes

mites and its precursors were characterized by X-ray diffrac- Method of temperature OH/Altion (XRD), thermal analysis (DTA/TGA), solid state 27Al dosing (7C) ratio Precursor a Boehmiteb

MAS NMR, and transmission electron microscopy (TEM).Simultaneous 25 2.7 Si-25-2.7 Si-25-2.7/AThe information from these physical characterization tech-Simultaneous 25 3.1 Si-25-3.1 Si-25-3.1/Aniques have been related to the peptization behavior of theSimultaneous 25 5.4 Si-25-5.4 Si-25-5.4/A

boehmite products. In addition, although the chemistry of Simultaneous 85 2.7 Si-85-2.7 Si-85-2.7/Athe hydrolyzed aqueous solutions of Al3/ is very well docu- Simultaneous 85 3.1 Si-85-3.1 Si-25-3.1/A

Simultaneous 85 5.4 Si-85-5.4 Si-85-5.4/Amented, studies on the nature of fresh aluminum gels formedSequential 25 2.7 Se-25-2.7 Se-25/Acupon hydrolysis to ratios above 2.5 are scarce. The presentSequential 25 3.1 Se-25-3.1work deals with these amorphous gels and may also contrib-Sequential 25 5.4 Se-25-5.4

ute to this field. Sequential 85 2.7 Se-85-2.7 Se-85/Ac

Sequential 85 3.1 Se-85-3.1Sequential 85 5.4 Se-85-5.4EXPERIMENTAL

a The code refers to the precipitation conditions, e.g., Si-25-3.1 indicatesMaterialssimultaneous dosing at 257C and OH/Al Å 3.1.

b The extension ‘‘/A’’, meaning ‘‘aged,’’ serves to differentiate theA 1.5 M aluminum nitrate solution was prepared withboehmite from the corresponding freshly precipitated precursors.Al(NO3)3r9H2O 98/% supplied by Fluka AG. Ammonium

c In the sequential dosing, the final boehmite is the same. Precursorshydroxide (8.3 M) was prepared from a concentrated ammo-(same three OH/Al levels as in simultaneous dosing) are collected during

nia solution (28% NH3, BDH Inc.) and standardized with the reaction.1.0 M oxalic acid. The nitric acid solution was prepared bydiluting concentrated HNO3 (BDH Inc.) .

Two different temperatures were used as variable parame-ters in the neutralization step: 25 { 2 and 85 { 27C. TableSynthesis Procedure1 summarizes the experimental parameters.

Boehmites were synthesized by neutralizing an aluminum A Ag/AgCl combination pH electrode connected to anitrate solution with NH4OH under various conditions, fol- Cole–Parmer pH–mV meter with an automatic temperaturelowed by an aging step at high OH/Al ratio and temperature. compensation probe were used for pH and temperature mea-Two methods of adding the reactants were used in the neu- surements. A lid was placed on the beaker for high tempera-tralization step: ture reactions to avoid evaporation. High dispersion of the

( i ) Sequential dosing (variable pH) . An 8.3 M ammo- solution in the beaker was obtained with the aid of a motor-nia solution was added dropwise into a 2-liter beaker con- driven four-blade propeller operating at maximum stirringtaining 200 ml of 1.5 M Al(NO3)3 and 250 ml distilled water rate.at a constant rate of 5.0 ml/min until OH/Al Å 5.4. These For sequential dosing, despite vigorous mixing, the for-samples are denoted as Se– N– , where N refers to the tem- mation of some insoluble ‘‘flocs’’ could not be avoidedperature of the reaction. at the outset of the reaction due to the high concentration

of the reactants. At an OH/Al ratio of 2.6, irrespective of( ii ) Simultaneous dosing (constant pH) . The sameamount of the ammonia and aluminum nitrate solutions as the temperature, these flocs rapidly became indistinguish-

able since the solution quickly thickened to a highly vis-used in the sequential dosing were added simultaneously ata constant rate into a beaker containing 250 ml of distilled cous gel. This coincided with a steep jump in the pH.

After an OH/Al ratio of 3.0 was reached, the viscositywater. The rate of addition of the ammonia solution wasmaintained at 5.0 ml/min as before, whereas the aluminum decreased, and remained nearly constant from this point

on. The dependence of the pH on the OH/Al ratio asnitrate addition rate was set according to the desired OH/Al ratio, namely 2.7, 3.1, and 5.4. After complete dosing of measured in the sequential dosing reactions is displayed in

Fig. 1. The three arrows show the points where precursorsaluminum nitrate at OH/Al ratio 2.7 or 3.1, a final adjust-ment to OH/Al Å 5.4 was carried out by adding additional samples were collected at OH/Al Å 2.7, 3.1, and 5.4 for

comparison with the corresponding simultaneously dosedammonia. These samples are denoted Si– N–R , where N isthe temperature of reaction, as above, and R refers to the precursors. The aliquots were immediately frozen at

01967C for subsequent freeze–drying at 1004 bar.OH/Al ratio (2.7, 3.1, or 5.4) used during the simultaneousdosing. In the case of the simultaneous dosing reactions, a hazy

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259FORMATION OF PEPTIZABLE BOEHMITES

peptization tests which can be classified with respect to theirbehavior:

( i ) the suspension changed dramatically to a clear orslightly translucent sol after acid treatment ( these will bereferred to as peptizable boehmites (class P));

( ii ) the suspension was visibly transformed to an opaque-translucent colloidal dispersion that showed little or no set-tling (these are the partly peptized boehmites (class PP));

( iii ) the suspension showed no significant transformationafter acid treatment and the suspension partially settled afterfew minutes (these will be referred to as nonpeptizableboehmites (class NP)) .

The synthesized boehmites gels, stored in closed contain-ers, were quite stable, showing the same peptization behavior

FIG. 1. Dependence of pH on the hydrolyis ratio in the neutralization between the first day and 30 days of storage.reaction of aluminum nitrate by ammonia: Solid markers, 257C; open mark-ers, 857C. Particle Size Measurements

Dynamic light scattering was used to determine the evolu-tion of the particle sizes of the peptized boehmite sols withsuspension was formed at the beginning of all experiments.time. An aliquot of the peptized system was collected andA slight viscosity increase occurred during the reaction butdiluted with millipore filtered distilled water to lower thanwas not as marked as in the sequential dosing. The pH of0.05% solids to give the appropriate scattering counts foreach simultaneous neutralization reaction corresponds to thatthe measurement. A Brookhaven instrument (Model BI-extrapolated from the respective OH/Al ratios (vertical200SM) was set up to determine the autocorrelation functionarrows) in Fig. 1. At the start of the dosing, the pH variedof the scattered intensity at 907 to the incident beam. Thewithin {0.2, but soon stabilized within {0.05 or less. Afterlight source was a Lexel-95 argon ion laser with l Å 514addition of all the aluminum nitrate solution, precursor sam-nm, operating typically at 50 mW. A photomultiplier tubeples were collected and freeze–dried as described above. Inreceived the scattered light at 907 after being collimatedthe case of OH/Al Å 2.7 and 3.1, addition of NH4OH wasthrough a 100-mm pinhole. An interference filter was usedcontinued after collection of the precursors until a final OH/to prevent room light from reaching the detector. The pulsesAl of 5.4 was reached.were counted and sent to a computer associated digital auto-After the precipitation step, the suspensions formed atcorrelator, which processed the data. The effective diameterOH/Al Å 5.4 were heated to 857C (when required) andand polydispersity, calculated from the second moment cu-stirred at this temperature for 1 h in a covered beaker in ordermulant analysis (17), were used as a measurement of theto form the boehmite phase. The final slurry was filteredmean particle size (MPS) and the relative width of the distri-and the resulting filter cake was washed to remove residualbution, respectively. Particle size distribution (PSD) calcula-NH4NO3. Part of the wet filter cake (84–93% H2O) wastions, using a nonnegatively constrained least squares proce-dried at room temperature under vacuum (1004 bar) fordure (NNCLS), generally showed either a multimodal orpowder analyses; the remainder of the boehmite gel wasbroad distribution for all peptized systems. The PSD resultskept in closed containers for the peptization tests.are not considered in this study, due to a considerable lackof reproducibility.Peptization Tests

Sonication ExperimentsThe boehmite gel was redispersed in distilled water sus-In order to determine the susceptibility of boehmite gelspension until complete homogenization occurred to give a

to physical disaggregation other than acid peptization, the4 wt% Al2O3. The pH of the resulting suspension was similarunpeptized suspensions (4%Al2O3) were sonicated using afor all samples (8.4 { 0.4) . The viscosity of the suspensionsCole–Parmer sonicator, (Model 8890MTH) and aliquots forat this pH varied from sample to sample. Generally, vigorousparticle size measurements were collected as describedstirring was required to keep the slurry agitating. A nitricabove.acid solution (HNO3 5 M) was then added dropwise (ca. 2

ml/min.) to the suspension until a pH of 2.00 was reached.X-ray Powder DiffractionAt this point a sequence of particle size and pH measure-

ments was made as a function of time. X-ray powder diffraction patterns for all precursors andboehmites samples were obtained on a Siemens D500 X-rayThree main classes of boehmites were observed in these

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diffractometer equipped with a diffracted beam monochro- erating at 200 KV. Samples were prepared by dipping aholey carbon copper grid into dilute (1 to 4% solids) suspen-mator operating at 40 kV and 30 mA using Cu Ka radiation.

Typically, a scanning velocity of 37 /min and a scan time of sions. A good specimen deposition on the carbon film wasnot possible for peptizable samples. A strong shrinkage dur-1 s per step were used to scan the four main peaks of the

boehmite difraction pattern in the 2u range between 2 and ing the drying of the gel damaged the carbon support andgenerated thick ‘‘glass-like’’ aggregates that could not be557. Semiquantitative analysis was performed by integration

of the areas under the four main peaks at 2u Å 14, 28, analyzed. The edges of some remaining particles supportedon the metalic grid were analyzed in these cases.38, and 497. The relative crystalline material content in the

samples (%boehmite) was determined by comparing thesum of the areas to that of a reference sample of crystalline RESULTS AND DISCUSSIONboehmite (Pural 200 from Condea GmbH) arbitrarily con-sidered to be 100% boehmite. By measuring the broadening Boehmitesof all the peaks (FWHH), the average crystallite size (CS)was obtained by using the classical Scherrer equation. The Peptization behavior. The peptizability (as defined

here) is the susceptibility of the boehmite gel or precipitaterelative error for the sum of integrated areas and calculatedcrystallite sizes were similar and estimated to be approxi- to form a sol when treated with nitric acid, as measured by

the mean particle size of the sol. Depending on the naturemately 5% by running one sample several times with a newrepacking of the sample for each measurement. of the boehmite, different amounts of acid were used to

obtain the same initial peptizing pH. Table 2 summarizesDifferential and Thermogravimetric Analysis the particle size obtained after peptization for the complete

series of boehmite gels. The polydispersity figures (notAn STA-1500 apparatus (PL Thermal Sciences) was usedshown) were in the range of 0.18–0.28 although a fewto record the differential thermal analysis and weight lossshowed higher values up to 0.50. These numbers indicate apatterns of boehmite and the precursors. The freeze–driedrelatively broad distribution compared to standard monodis-gel precursors were washed with excess distilled water andperse systems that show values less than 0.05.oven-dried at 907C overnight before DTA/TGA measure-

Peptizable boehmites (class P) were obtained only by lowments to remove residual ammonium nitrate that would hin-temperature precipitation. The clear sols obtained after 1 hder the interpretation of the aluminum hydroxide thermalof peptization contained particles smaller than 80 nm. Highbehavior. DTA and TGA curves were run simultaneouslytemperatures combined with high OH/Al ratios (ú3.0) ledon each Ç15-mg sample from room temperature to 9007Cto nonpeptizable boehmites (class NP), which upon acidifi-in a flowing atmosphere of air at 20 cm3/min using a heatingcation did not produce a submicrometer colloidal dispersion.rate of 207C/min.Boehmites that were not completely peptized (class PP)gave submicrometer particles not lower than 200 nm, charac-27Al MAS NMRteristic of a hazy but homogeneous dispersion. These materi-

Solid-state 27Al MAS NMR spectra of the hydroxide pre- als were obtained by high temperature precipitation at lowcursors and some boehmite gels were acquired at 130.3 MHz OH/Al ratios. It should be emphasized that in the sequentialon a Bruker AMX-500 spectrometer with complete solid dosing the precipitation mostly occurs at low, though vari-state accessories using a Doty MAS (magic angle) high- able, OH/Al ratios, starting from pH Ç 2.spinning probe. The freeze–dried powders were oven- The boehmite gels converted to submicrometer disper-treated at 707C for 1 h, ground to smaller than 250 mesh, sions after sonication, except for the nonpeptizable boeh-loaded into 5 mm cylindrical zirconia rotors, and spun at 12 mites. The particle size, however, did not decrease to lowerkHz. A radiofrequency pulse width of 0.5 ms (307) and a than ca. 200 nm. It seems that the peptizability is related todelay time of 1 s between pulses were used to accumulate the susceptibility to physical disaggregation upon sonication.32 free-induction decays. Chemical shifts were established An exception is the sample Si-85-2.7/A, where no furtherby using 1 M [Al(H2O)/3

6 ]Cl3 as the external reference (0 disaggregation ocurred during acid treatment. This suggestsppm). Chemical shifts were assigned from peak maxima that the agglomerates in the peptizable and partly peptizable(not the true isotropic shift) . Integration of the peaks were gels are loosely bound and thus generate submicrometer sec-carried out using WinNMR software to give an estimation of ondary particles as small as 200 nm under low power sonica-the relative content of octahedral and tetrahedral aluminum tion. These secondary particles are further reduced in sizespecies when the latter were present. by acidic peptization for samples of class P, whereas they

are more resistant to further chemical disaggregation in sam-Transmission Electron Microscopy

ples of class PP. Nonpeptizable gels, conversely, havestrongly agglomerated structures, resistant to both physicalBoehmite gels were examined by TEM and selected area

diffraction (SAD) using a Philips CM 20 instrument op- and chemical disaggregation.

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TABLE 2Peptization of the Boehmite Gels

Mean particle size (nm)Acidd consumption

Sample Classa Unpeptized After sonication b After peptizationc mol HNO3/AlOOH

Si-25-2.7/A P ú1000e 716 70 0.11Si-25-3.1/A P ú1000 200 47 0.12Si-25-5.4/A P 965 197 52 0.12Si-85-2.7/A PP ú1000 240 306 0.10Si-85-3.1/A NP ú1000 ú1000 ú1000 0.09Si-85-5.4/A NP ú1000 ú1000 ú1000 0.08Se-25/A P ú1000 225 35 0.12Se-85/A PP ú1000 680 225 0.20

a Peptization behavior: P, peptizable; PP, partly peptizable; NP, nonpeptizable.b Sonication for 20 min at low power.c Particle size measured after 60 min of peptization (initial peptizing pH 2.0).d Amount of nitric acid required to adjust the pH to 2.00.e Particles larger than 1 mm did not allow a good fit of the autocorrelation function and hence the figures are not reliable.

Figure 2 shows that peptization is a dynamic process pH increase that tends to stabilize near 4.0. A remarkabledifference in the behavior of the partially peptizable boeh-which over time results in particle size reduction and pH

increase. Small differences in the degree of peptization were mite Se-85/A is the much faster increase in pH upon peptiza-tion. The consumption of acid (Table 2) is also considerablyfound among the peptizable boehmites. Sample Si-25-2.7/

A gives a sol with slightly larger particle size than its coun- greater for this sample, which coincides with a constantparticle size after acidification. It suggests a rapid dissolutionterparts prepared at higher OH/Al ratios, Si-25-3.1/A andof some less crystalline material that rapidly releases second-Si-25-5.4/A. Boehmite derived from the sequential dose ex-ary particles. The resultant particles, however, are strongperiment (Se-25/A) led to the most easily peptized systemaggregates that are resistant to further peptization. No re-whose sol looked like a clear solution within 1 h of peptiza-markable behavior in acid consumption was found for thetion. The two partly peptizable boehmites Se-85/A and Si-other samples.85-2.7/A, have a particle size stabilized around 200 nm;

however, the latter becomes stabilized only after 60 min of XRD data. The XRD data (Fig. 3) indicated that boeh-mite was the only crystalline alumina phase formed. Thepeptization. The gradual consumption of H3O

/ leads to a

FIG. 2. Mean particle size (MPS) and pH behavior with time in the peptization of P and PP type boehmites; markers in the plot of pH ratios timecorrespond to those used in the plot of MPS ratios time.

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(Se-25/A) was the same as its simultaneous dose counter-part at low temperature. The most distinct characteristic wasfound for sample Se-85/A, which had a boehmite contentconsiderably lower than the other samples, even though itsroughly estimated crystallite size was the greatest. Thehigher amount of amorphous material found for this materialis clearly responsible for the rapid and high consumption ofacid observed in the peptization tests. No marked relation-ship was found between peptizability and the XRD features.Peptizable boehmites, however, appear to have slightlysmaller, more symmetric crystallites than the PP and NPcounterparts. XRD measurements on the freeze–dried pep-tized materials showed that the crystallinity of the boehmitesgels was preserved after the acidic peptization.

DTA/TGA. Differential thermal analysis curves areshown in Fig. 4 and the weight loss data is summarized inTable 4. In all samples, two main endothermic peaks couldbe distinguished. The low temperature endotherm centeredat about 1007C can be attributed to loosely bound sorbedwater. The intensity of this peak varied somewhat, due tothe variations in the freeze–drying step of the boehmite gels.The second thermal feature spans the region from 250 and

FIG. 3. X-ray diffraction patterns of the boehmite samples.5307C and is attributed to the dehydroxylation process. Nosignificant weight loss was observed beyond 5307C. In gen-eral, the first peak overlapped with the higher temperaturedata are summarized in Table 3. A small shift (ca. 17) of

the first peak (020 reflection) to lower 2u values, relative peak, this effect being more evident for boehmites precipi-tated at 257C. In some samples an intermediate small broadto well crystalline boehmite, was observed for all samples.

This feature is expected for poorly crystalline boehmites band was observed at about 2207C, which was quite promi-nent in sample Se-85/A. We tentatively assign this to(2) . The XRD patterns of the boehmites synthesized by

simultaneous dosing were all similar and small differences strongly bound water associated with a transitional (amor-phous) aluminum hydroxide.in their relative boehmite contents were within the experi-

mental error (Table 3). The calculated average CS, however, For the simultaneous dose derived boehmites, no signifi-cant effect of the OH/Al ratio used in the precipitation stepwas slightly higher for the boehmites precipitated at higher

temperatures mainly due to the relative narrowing of the was observed. The dehydroxylation endotherm of theboehmites precipitated at 857C, increased in intensity, how-second and fourth peaks, assigned to the (120) and superim-

posed (051)/(200) reflections, respectively. ever, and slightly shifted to higher temperatures relative tothe room temperature prepared samples (see Table 4); thisThe XRD pattern of the sequential dose derived boehmite

TABLE 3XRD Results for the Boehmites

Crystallite size (nm)

Samples % Boehmite (020)a (120)a (140/031)a (051)/(200)a Average

Si-25-2.7/A 70 3.2 3.3 3.4 4.0 3.5Si-25-3.1/A 67 2.9 3.3 3.7 3.8 3.4Si-25-5.4/A 69 2.6 3.3 3.4 4.0 3.3Si-85-2.7/A 69 3.0 4.5 4.0 6.7 4.5Si-85-3.1/A 71 3.1 4.1 3.5 6.4 4.3Si-85-5.4/A 73 3.0 4.0 4.5 5.8 4.3Se-25/A 67 3.0 4.0 3.6 3.8 3.6Se-85/Ab 35 4.9 5.1 5.2 7.1 5.5

a Miller index.b Estimation of the crystallite size for this particular sample is subject to a larger error due to the very low peak intensity.

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FIG. 4. DTA patterns of the boehmite samples.

is an indication of a small increase in crystallite size (10, slightly dependent on the selected region (thickness effect) ,but no significant differences could be found among the18) and is in agreement with the XRD data. As in the latter,

the thermal analysis results showed no correlation to the samples.Bright and dark field images were examined. Dark fieldtype of peptization class of the boehmites.

images showed the presence of crystallites of similar sizes,TEM results. Boehmite gel specimens representing eachat least of the same order of magnitude, as those calculatedclass of peptization behavior were studied by TEM to deter-from the XRD pattern. However, in some cases, distortionsmine if their microstructure could account for their differentin the image due to ‘‘charging ’’ effects led to an unaccuratepeptizabilities. At a general level, the samples showed theanalysis. The bright field images of some boehmite gels aresame diffuse-ring electron diffraction pattern characteristicshown in Fig. 5.of microcrystalline boehmites. The diffraction patterns were

Nonpeptizable (NP) boehmites (Figs. 5a and 5b) showedthe formation of randomly interconnected fibrils, whoselength could not be readily determined. Sample Si-85-3.1/

TABLE 4A contained thicker agglomerates organized as bundles of

DTA/TGA Measurements of Boehmitesfibrils; hence individual crystallites were not visible. Theimage obtained for sample Si-85-5.4/A suggested the pres-Weight loss (%)

TOH Residue ence of small crystallites that were linked to form the boeh-Sample (7C) a 30–2507C 250–5307C 530–9007C (%) mite fibrils. In contrast, peptizable boehmites gels (P), such

as sample Se-25/A (Fig. 5c) , displayed no specific morphol-Si-25-2.7/A 376 11.9 12.9 1.6 73.6

ogy. The crystallites are agglomerated and distributed in aSi-25-3.1/A 372 21.1 11.4 2.1 65.3homogeneous fine and porous matrix. Sample Si-25-3.1/ASi-25-5.4/A 387 20.4 11.0 1.6 67.1

Si-85-2.7/A 410 14.2 12.4 1.6 71.9 showed similar features. The TEM image of partly peptiza-Si-85-3.1/A 405 13.1 12.6 1.6 72.7 ble boehmite (Se-85/A; Fig. 5d) displayed characteristicsSi-85-5.4/A 393 15.8 13.1 1.8 69.2 between NP and P type boehmites, i.e., a background ofSe-25/A 358 14.2 11.7 1.5 72.6

homogeneous matrix of crystallites with some visible fibrilsSe-85/A 389 18.5 16.4 1.5 63.4in the early stages of formation. The same general feature

a Temperature of the dehydroxylation peak maximum. was found for the PP type boehmite Si-85-2.7/A.

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FIG. 5. Bright field images (TEM micrographs) of boehmite samples with different peptizabilities: (a) Si-85-3.1/A (NP), (b) Si-85-5.4/A (NP),(c) Se-25/A (P), (d) Se-85/A (PP).

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FIG. 6. DTA patterns of the precursor samples.

Precursors Fig. 4, which is in accordance with the XRD results. Theweight loss was also similar to the final boehmites. The other

Dissolution tests. All the precursors (freeze–dried pow-precursors showed similar, indistinguishable patterns. A ma-

ders) , with the exception of the samples Si-85-3.1 and Si-jor broad endothermic peak, centered at about 2207C, indi-

85-5.4 (precursors of nonpeptizable boehmites) , were easilycates the presence of strongly bound water. We assign thesoluble in nitric acid below pH 2.5, at room temperature.small shoulder at about 330–3507C to water loss from struc-The two insoluble materials could not be completely dis-tural hydroxyl groups in the precursor. A continuous loss ofsolved even at pH õ 1. Sample Se-85-2.7 was almost com-water up to 5307C was observed in the thermogravimetricpletely soluble in water at room temperature; for this reason,curve (not displayed) for all amorphous precursors. Theit was not possible to wash the material for DTA/TGAresidue at 7007C for all these samples (62–64%) was lowermeasurements.than those of the corresponding boehmites (Table 4), as

XRD data. None of the precursors contained a crystal- expected.line aluminum hydroxide phase except for samples Si-85-

The DTA/TGA patterns of Se-85/A are close to those of3.1 and Si-85-5.4, which contained boehmite. Residual

its amorphous precursors, showing the same broad peak atNH4NO3 in all samples was identified from its diffraction2207C, which indicates that a high amount of amorphouspattern. In order to determine the relative boehmite contentmaterial still remained in this boehmite sample. Neverthe-in the crystalline precursor, the fresh gel precursor Si-85-less, the aged material showed a more prominent dehydroxy-3.1 was filtered and washed before the freeze–drying steplation endotherm.to remove NH4NO3. The XRD analysis indicated a typical

27Al MAS NMR. The amorphous nature of the boehmiteboehmite pattern corresponding to 56% crystalline material,precursors precludes the use of diffraction techniques. Solidcompared with 71% crystalline material for the correspond-

ing boehmite after aging (Si-85-3.1/A; Table 3). Hence, state NMR spectroscopy, however, provides useful informa-most of the crystallization took place in the precipitation tion on these materials which is specific to the Al coordina-step and a slow crystallization rate was observed in the aging tion site(s) , thus allowing to work on the fresh precursorsstep. This is in sharp contrast with the behavior of the XRD- without any washing step. The spectra of the precursors areamorphous hydroxide precursors. presented in Fig. 7. The chemical shift and linewidths are

summarized in Table 5, along with the relative proportionDTA/TGA data. Differential thermal analysis curves areof the tetrahedral and octahedral sites.shown in Fig. 6. Precursors Si-85-3.1 and Si-85-5.4 showed

a pattern similar to the corresponding ‘‘aged’’ boehmites in Tetrahedral aluminum sites were detected in the precur-

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FIG. 7. Solid state 27Al MAS NMR spectra (130.3 MHz) of the boehmite precursors.

sors prepared by sequential dosing (Se-) (Fig. 7a) . These actions (Si-) contained no tetrahedral species (Fig. 7b), ex-cept for the precursor Si-85-2.7, which has a spectrum veryaccounted for up to 7% of the total Al content at OH/Al Å

3.1 for materials prepared at 857C. In the low temperature similar to its counterpart prepared by sequential dosing (Se-85-2.7, Fig. 7a) . The precursors Si-85-3.1 and Si-85-5.4samples, however, tetrahedral Al was only present to an

extent of less than 0.5% of the sample. showed a single octahedral resonance with a linewidth twiceas narrow as the other amorphous precursors, suggesting aThe precursor gels derived from simultaneous dosing re-more uniform octahedral enviroment for this material, whichis in line with their XRD crystalline nature.

TABLE 5 The NMR spectra of the boehmite gels Se-85/A and Si-27Al MAS NMR Data of Aluminum Hydroxide Precursors 85-2.7/A, whose precursors also contained tetrahedral alu-

minum, are shown in Fig. 8. It is evident that all of the Al IVChemical shift Relative

from the precursor Si-85-2.7 was transformed to AlVI after(ppm) content (%) Oh

Linewidth aging (Table 5) and crystallization to boehmite. The dis-Sample Oh Td Oh Td Hz torted tetrahedral aluminum sites in the Se-85 precursor were

still visible in the poorly crystalline boehmite Se-85/A.Se-25-2.7 11.2 68 — ??a 1900Se-25-3.1 10.6 67 99.6 0.4 1820

Mechanism of the Formation of the Al GelsSe-25-5.4 12.4 67 99.6 0.4 1600Se-85-2.7 12.1 67 96.7 3.3 1745

The base hydrolysis of Al/3 aqueous solutions (19) atSe-85-3.1 10.9 68 93.1 6.9 2468Se-85-5.4 12.7 67 95.6 4.4 1530 OH/Al ratios between 0.5 and 2.5 is known to produce theSi-25-2.7 10.6 — 100 0 1975 tridecameric ion, [AlO4Al12(OH)24(H2O)12]7/ or ‘‘Al13’’Si-25-3.1 11.4 — 100 0 1715 composed of a central highly symmetric tetrahedrally coordi-Si-25-5.4 11.7 — 100 0 1710

nated aluminum atom, surrounded by 12 octahedrally coordi-Si-85-2.7 12.2 67 97.3 2.6 1888nated Al sites. If hydrolysis progresses to OH/Al ratios be-Si-85-3.1 13.4 — 100 0 910

Si-85-5.4 13.3 — 100 0 930 tween 2.5 and 3.0, an amorphous gel phase is formed proba-Se-85/A 13.1 68 98.7 1.3 1125 bly via the consolidation of these tridecamers (20, 21) inSi-85-2.7/A 13.3 — 100 0 945 addition to other species (e.g., monomers) . However, the

formation of the Al13 ion is very sensitive to the hydrolysisNote. Oh , octahedral site; Td , tetrahedral site.conditions (22–24) and this precursor may not be present;a Integration hindered by overlapping of the spinning side band at lower

spinning rates (10 kHz). therefore, an alternative mechanism might involve the con-

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avoided. The literature would suggest that base hydrolysisin our sequential dose conditions would present a very unfa-vorable medium for the formation of the Al13 ion. Neverthe-less, tetrahedral aluminum was found in gels, particularlythose formed at high temperature. We believe that once theAl(OH)04 is locally formed, it is rapidly encapsulated bysoluble aluminum octahedral species to form Al13 species.At high temperatures, either this encapsulation process isfavored and/or the lifetime of the fourfold coordinated ionis longer. In the case of simultaneous dosing, however, thebase is added to a medium with a relatively low concentra-tion of Al(H2O)3/

6 and is very rapidly consumed, dependingon the OH/Al ratio. In this case the absence of tetrahedralAl was consistent with the relatively high OH/Al ratios(ú2.5) and with the Bertsch mechanism (23), since thesolution would be relatively starved in Al(H2O)3/

6 , andhence there would not be a sufficient excess of octahedralspecies to react with any Al(OH)04 that was generated. Thelatter complex would be rapidly converted to octahedral spe-cies which could form a gel through condensation reactions.The only exception was Si-85-2.7, where the simultaneousaddition occurred at lower OH/Al ratios (higher excess ofAl(H2O)3/

6 ) and higher temperature (possible longer life-time for the Al(OH)04 ion).

Once some Al13 is formed as a precursor in solution, wepropose that the resulting amorphous gel is built by consoli-dation or agglomeration of the tridecameric ion. NMR spec-tra of the gel precursors displayed a tetrahedral Al resonanceat a chemical shift of 67–68 ppm, higher than that in theFIG. 8. Solid state 27Al MAS NMR spectra (130.3 MHz) of the boeh-

mite samples Se-85/A and Si-85-5.4/A; the spinning side bands (SSB) are solid Al13-sulfate (63 ppm). The upfield shift can be ascribedalso indicated. to a consequence of the coalescence of the tridecameric spe-

cies, as observed by Fu et al. (28), who observed an upfieldchemical shift in the dimerization of the tridecamer (63 r

solidation of the hexamer ring species (25, 26) which con- 70 ppm). A third larger species (unassigned) exhibited antains only octahedral Al enviroments. even higher d Å 75 ppm. It is, however, interesting to note

It is assumed by several authors (22–24) that that Bradley et al. (21) proposed a pseudospinel type of gelAl(OH)04 ions, formed inhomogeneously at the introduction (hydrated Al2O3) as an intermediate structure during theof base (supersaturation), are needed as a precursor for the transformation of an amorphous gel (containing tetrahedralformation of Al13 . However, Kloprogge and co-workers have species) to pseudoboehmite in the latter stages. The similar-found that at high base injection rates the Al13 concentration ity between the chemical shift of 67–68 ppm observed intends to decrease (24). An explanation suggested by Bertsch our freshly Al gel and that of h-Al2O3 spinel (d Å 68.1 ppm)(23) is that more Al(OH)04 is generated than can be con- (29) is also consistent with their mechanism.sumed by the formation of Al13 . The excess of Al(OH)04 It is known that the concentration of Al13 in solution dra-reequilibrates with the bulk solution, resulting in some col- matically decreases above OH/Al Å 2.5, following the for-loidal or precipitated Al(OH)3 species. Kloprogge et al. (27) mation of insoluble aluminum species (24, 30). Generally,have also reported the absence of the tridecameric ion in it is assumed that no tetrahedrally coordinated aluminum ishydrolyzed solutions of high aluminum concentrations and present when hydrolysis has advanced to an OH/Al ratio ofproposed an explanation based on the Al(OH)04 mechanism: 3.0. Indeed, Bottero et al. (20) have observed the formationnamely, an increase of Al concentration would result in less of highly polymerized octahedral layers and the virtual dis-or no supersaturation at the point of the base introduction, appearance of tetrahedral aluminum for OH/Al ratios Å 3.0,thus preventing the formation of Al(OH)04 and hence Al13 . in colloidal samples obtained by partial hydrolysis of AlCl3

In the present study, the base injection rate was relatively solution. In contrast, we have observed the presence of tetra-high, as were the aluminum and base concentrations; the hedral species up to OH/AlÅ 5.4, particularly from hydroly-

sis at high temperatures.formation of insoluble aluminum hydroxides was not

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FIG. 9. Scheme of the proposed mechanism for the formation of microcrystalline boehmites with different peptizabilities obtained from the neutraliza-tion of aqueous aluminum nitrate solutions by ammonia.

Boehmite Synthesis vs Peptization Behavior observed for the P type boehmites. In this gel, however, thecrystallites are aggregated into larger secondary particlesPeptizable microcrystalline boehmites (P type), are ob-having a fibrillar morphology and are strongly intercon-tained by neutralization of aluminum nitrate solutions atnected. This is undoubtedly the cause of the high resistanceroom temperature, irrespective of what procedure is used.of this material to peptization.Either using variable (sequential) or constant pH (simultane-

PP type boehmites were obtained when the OH/Al ratioous) batch conditions, amorphous AlO(OH) precursors arewas maintained below 3.0 during precipitation at 857C. Informed with virtually 100% octahedral aluminum content.this case, amorphous gel precursors with partial tetrahedralThese precursors gave rise to a microcrystalline boehmitealuminum content were formed. Although their amorphousgel (ca. 70% boehmite content) after aging at 857C for 1 h,content was different, both of the PP boehmites exhibitedwhose Ç4-nm crystallites were not connected to form anythe same morphology: namely, poorly defined fibrils thatspecific secondary microstructure; that is, they were looselyappeared to be in the intermediate stages of formation. Thisbound in a single agglomerate. This aggregrate was thusintermediate behavior between P and NP morphologicaleasily disrupted into smaller colloidal unities by sonicationtypes indicates that some condensation of the boehmite nano-and further disaggregated into a clear sol after peptization.crystallites (primary particles) into secondary particles leadsAt a higher neutralization (precipitation) temperature ofto a loss of peptizability of the corresponding boehmite gel.857C, PP and NP boehmites were obtained. The latter type

The presence of a considerable amount of tetrahedrallywas obtained when the OH/Al ratio was maintained abovedistorted aluminum in the structure of the PP type precursors3.0 during the precipitation step. Here, a fast condensationgels may hinder the rapid crystallization of boehmite (whichprocess is favored, resulting in the premature crystallizationrequires the presence of only sixfold coordinated Al) in theof boehmite. This crystalline precursor did not undergo re-precipitation step. This results in a lesser degree of aggrega-markable transformation during the aging step and led to a

boehmite gel with an XRD pattern that was similar to those tion/condensation of the primary particles into larger fibrils

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and thus retention of some degree of peptizability in the ACKNOWLEDGMENTSresulting boehmites.

This research was made possible by a grant from Petrobras SA to E.M.J.Despite the very similar NMR spectra shown by the amor-Facilities used in the University of Waterloo were provided by NSERC,phous precursors Se-85-2.7 and Si-85-2.7, different boeh-and the research was supported through a NSERC grant to L.F.N. The

mite gels were obtained from them after the same aging step. authors gratefully acknowledge Prof. J. Corbett and M. Robertson (Univer-A boehmite with a lower degree of XRD crystallinity and sity of Waterloo) for obtaining the TEM images. We would thank Prof.

M. Gauthier (University of Waterloo) and C. M. C. Figueiredo (Petrobrassome unconverted AlIV was obtained in the former case.Research Centre) for their invaluable assistance. We also are grateful toThis may be the result of a higher degree of disorder of theDr. S. Mooibroek for acquiring the NMR spectra.Al IV within the AlO(OH) precursor structure that impedes

the complete conversion to AlVI and transformation toREFERENCESboehmite during aging, compared to the precursor Si-85-2.7.

A proposed representation of the mechanism of formation1. Baker, B. R., and Pearson, R. M., J. Catal. 33, 265 (1974).

of boehmite gels of different peptizabilities is depicted in 2. Tettenhorst, R., and Hofmann, D. A., Clays Clay Miner. 28(5) , 373Fig. 9. (1980).

3. Yoldas, B. E., Am. Ceram. Soc. Bull. 54, 289 (1975).4. Hille, J., Bollmann, U., Weinhold, W., and Spindler, H., Z. Anorg.

CONCLUSIONS Allg. Chem. 580, 188 (1990).5. Vlaev, L. T., Ivanov, I. V., Damyanov, D. P., and Dzisko, V. A., Kinet.

Katal. 34(1) , 147 (1993).The peptization characteristics of a boehmite gel depend6. Jiratova, K., and Janacek, L., Chem. Prumysl. 31, 174 (1981).on the precipitation conditions of the precursor aluminum7. Trimm, D. L., and Stanislaus, A., Appl. Catal. 21, 215 (1986).

hydroxide. In the synthesis based on the neutralization of 8. Strenge, K., and Bollmann, U., Colloids Surf. 57, 139 (1991).aluminum nitrate solutions by ammonium hydroxide, high 9. Scherzer, J., and Liu, A., US patent 4,332,782 (1982).

10. Zubowa, H. L., Becker, K., Hennig, H. P., Kretzschmar, U., Noack,hydrolysis temperatures (857C) do not favor the formationM., and Steinike, U., Chem. Tech. 41(9) , 387 (1989).of peptizable boehmites. Premature crystallization occurs,

11. Oberlander, R. K., and Decker, L. B., Jr., US patent 4,360,449 (1982).resulting in strong aggregation of crystallites into large inter-12. Lippens, B. C., Ph.D. thesis, Tech. Univ. Delft, 1961.

laced fibrillar particles. If this high temperature reaction oc- 13. Gitzen, W. H., ‘‘Alumina as a Ceramic Material.’’ American Ceramiccurs at hydrolysis ratios (OH/Al) lower than 3.0, however, Society, Columbus, OH, 1970.

14. Wefers, K., and Misra, C., ‘‘Oxides and Hydroxides of Aluminum,’’the crystallization and aggregation is hindered for the pres-Alcoa Technical Paper 19. Alcoa Laboratories, 1987.ence of fourfold aluminum species giving rise to a PP prod-

15. Hille, J., Bollmann, U., Weinhold, W., Becker, K., Bremer, H., anduct after aging. In the aging of the amorphous precipitateVogt, F., Z. Anorg. Allg. Chem. 579, 211 (1989).

produced by hydrolysis at room temperature, the boehmite 16. Prodromou, K. P., and Pavlatou-Ve, A. S., Clays Clay Miner. 43(1) ,nanocrystallites are formed as independent units without an 111 (1995).

17. Koppel, D. E., J. Chem. Phys. 57(11), 4814 (1972).appreciable degree of aggregation and thus lead to a boeh-18. Smothers, W. J., and Chiang, Y., in ‘‘Differential Thermal Analysis:mite gel easily disrupted by diluted nitric acid into a colloidal

Theory and Practice.’’ Chemical Publishing Company, New York,system with particles of ca. 50 nm.1958.

Both P and NP boehmites, as synthesized here, have a 19. Baes, C. F., and Mesmer, R. E., ‘‘The Hydrolysis of Cations.’’ Wiley,similar amorphous content (ca. 30%), reinforcing the state- New York, 1976.

20. Bottero, J. Y., Cases, J. M., Fiessinger, F., Tchoubar, D., and Fripiat,ment that peptizability is primarily determined by the degreeJ. J., J. Colloid Interface Sci. 117(1) , 47 (1987).of aggregation of primary particles. In peptizable boehmites,

21. Bradley, S. M., Kydd, R. A., and Howe, R. F., J. Colloid Interface Sci.the amorphous part would be homogeneously dispersed in159, 405 (1993).

the crystalline matrix, so that during reaction with nitric acid 22. Akitt, J. W., and Farthing, A., J. Chem. Soc. Dalton Trans. 1617a large number of small aggregates would be released. In (1981).

23. Bertsch, P. M., Soil Sci. Soc. Am. J. 51, 825 (1987).contrast, in the NP boehmites, the crystallites are already24. Kloprogge, J. T., Seykens, D., Jansen, J. B. H., and Geus, J. W., J.strongly interconnected, forming a crystalline frame resistant

Non-Cryst. Solids 142, 94 (1992).to disruption such that a possible reaction of the amorphous25. Smith, R. W., and Hem, J. D., U.S. Geological Survey Water-Supply

portion with the acid would not result in disaggregation. Paper 1827D (1972).The intermediate morphology of PP boehmites containing 26. Singhal, A., and Keefer, K. D., J. Mater. Res. 9(8) , 1973 (1994).

27. Kloprogge, J. T., Seykens, D., Jansen, J. B. H., and Geus, J. W., J.isolated groups of crystallites with a much lower degree ofNon-Cryst. Solids 160, 144 (1993).interlacing allows some disaggregation. The resultant mean

28. Fu, G., Nazar, L., and Bain, A. D., Chem. Mater. 3, 602 (1991).particle size in the colloidal acidic medium is larger than29. Plee, D., Borg, F., Gatineau, L., and Fripiat, J. J., J. Am. Chem. Soc.

those in peptizable boehmites due to the limitation imposed 107, 2362 (1985).by the secondary aggregates present in the gel microstruc- 30. Bradley, S. M., Kydd, R. A., and Yamdagni, R., J. Chem. Soc. Dalton

Trans. 2653 (1990).ture.

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Formation of Peptizable Boehmites by Hydrolysis of Aluminum Nitrate in Aqueous Solution

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Transcript of Formation of Peptizable Boehmites by Hydrolysis of Aluminum Nitrate in Aqueous Solution